Digital insulation meter measurement limits.

Discussion in 'General Electronics Chat' started by xchcui, Aug 9, 2017.

  1. xchcui

    Thread Starter Member

    May 12, 2014
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    Hi.

    There is a digital insulation meter,which one of its warning was:
    "When use 500V measurement to measure resistance less than 1.5MΩ or 1000V to measure resistance less than 5MΩ, measurement time must not exceed 10 seconds"
    The problem is that i need to make measurement beyond the 10 seconds.
    A measurement of 60 seconds and more.
    I assume that when the measured resistance is less than 1.5MΩ for 500V and less than 5MΩ for 1000V,i might be able to exceed the 10 seconds,but how many seconds/minutes can
    i make the measurment according to the measured resistance?
    For instance,if i measured 10MΩ with 500V,can i make the measurment for 60 seconds,3 minutes etc.?Is there a rule of thumb for this situation?

    Thanks.
     
    Last edited: Aug 9, 2017
  2. strantor

    AAC Fanatic!

    Oct 3, 2010
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    maybe ask the MFG why the 10s limit?
     
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  3. xchcui

    Thread Starter Member

    May 12, 2014
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    Hi,Strantor.

    I had already asked.No response from the manufacturer.
    This is the reason for ask it in this forum.
     
    Last edited: Aug 10, 2017
  4. xchcui

    Thread Starter Member

    May 12, 2014
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    Is there someone that expert with meggers?
     
  5. LesJones

    Active Member

    Jan 8, 2017
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    It could be that the inverter that generates the high voltage needs to provide more current with lower value resistances being tested. The components in the inverter may overheat if it is providing this higher current for a long time. I am assuming the this is a battery powered insulation tester rather than the old type windup megger.

    Les.
     
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  6. #12

    Expert

    Nov 30, 2010
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    That's where I was going, but you were here first.:(
    I think something will get too hot if you deliver too much power for too long.
    It would be convenient if the limit was described in watt seconds, but the MFG has declined to answer.:(

    500V into 1.5 meg suggests 1/6th of a watt (167 mw) for ten seconds is a limit.
    1000 V into 5 meg suggests 1/5th of a watt (200 mw) for ten seconds is a limit.
    I can't see inside the machine so I can't tell if it's a transistor or it's a resistor that over heats.

    Suppose you decided that half the power should last for twice as long?
    60 seconds would require an output power of 1/36th of a watt and 3 minutes would suggest 1/108 of a watt delivered to the load.

    10 meg and 500V? That's 25 milliwatts or 1/40th of a watt.
    167 mw/25 mw times 10 seconds = 66.66 seconds.

    That's one way, but no guarantees. If you really need a stable 1KV you can build a 1KV DC power supply.
    Been there, done that.
     
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  7. strantor

    AAC Fanatic!

    Oct 3, 2010
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    I have designed and built continuous running meggers (3 of them, still in service AFAIK) that keep a constant watch over the conductors within a cable as the cable is being made. They have digital outputs to stop the cabling machine and/or give audio visual alarms if IR drops below an adjustable threshold. My meggers did not suffer this 10s limit but they also ran on 120vac.
     
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  8. xchcui

    Thread Starter Member

    May 12, 2014
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    Yes,you're right.It is a battery powered megger.
    Thanks,#12.Your calculation makes sense,but while i made a measurment with
    1000V,checking resistance of 1MΩ for 8 seconds,Is it possible that i made any damage
    to the megger?Since if i use your calculation for 1MΩ and 1000V:
    1000V/1MΩ=1mA-->1mAx1000V=1000mw--->200mw/1000mwX10=2 seconds.
    It means that i shouldn't make the measurement of 1000v and 1MΩ more than 2 seconds,
    while i did it for 8 seconds(it took 8 seconds to the value resistance to stabilized),but in
    other hand,the MFG warning was as i said before"...1000V to measure resistance less than 5MΩ, measurement time must not exceed 10 seconds".
    So,how should i refer to this?sould i use your calculation only for time above 10 seconds,while i can measure any resistance that i like with 1000v and 500v,as long as
    it doesn't exceed the 10 seconds and the 2 seconds that i got with your calculation is not relevant in that situation?
     
  9. #12

    Expert

    Nov 30, 2010
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    I am not happy about the idea that you want all these things defined by (my) theory which nobody can prove.
    I tried to guess at what the real limits are and now you want a second level of guessing based on my guess.
    I don't think I want to do that today.
    I showed you how to guess. The rest is up to you, and that includes the option to buy the right machine for the job you want to accomplish.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
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    I am assuming that the digital ones have some kind of circuit limit also, I am used to the old Hand crank ones where there was no limit given.
    Never used the modern versions.
    Don't even know if they make them any more?
    Max.
     
  11. xchcui

    Thread Starter Member

    May 12, 2014
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    What did i ask?:confused:
    #12,i understood that you are just guessing.Your motto at the bottom of
    your post,also,clear up your intentions.
    I just made some calculations,based on your explanation and i wanted to verify if it is okay to measure any resistance with this megger(1MΩ,100KΩ,100Ω etc.),according to mfg warning,as long as it doesn't exceed the 10 seconds?
     
  12. xchcui

    Thread Starter Member

    May 12, 2014
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    I understand that,unfortunately,there isn't any expert about meggers
    in that forum.:(
    Anyway,thanks for trying.:)
     
  13. LesJones

    Active Member

    Jan 8, 2017
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    It is not that there are no experts. It is that you have not provided enough information for the answer to be any more than a guess. If you provided a full schematic of your insulation tester you would be more likely to get more than a guess as an answer. I think you should design your own insulation tester that meets your specific needs. You have not said exactly what you are trying to do but I suspect you are testing insulation on a wire manufacturing process.

    Les.
     
  14. strantor

    AAC Fanatic!

    Oct 3, 2010
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    :facepalm:
     
  15. xchcui

    Thread Starter Member

    May 12, 2014
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    I believe that if someone here was expert in meggers,a glimpse on the pcb photos
    were enough(due to the expert's experience)to know the answer.

    Anyway,thanks for your responses.
     
  16. MaxHeadRoom

    Expert

    Jul 18, 2013
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    You are missing the point, the term Megger is a generic one for Megohmer, originally derived from a certain manufacturer in the 1920's , they can take all kind of forms and design criteria in order to achieve the same result.
    Therefore no one can be completely conversant with all the ones produced.
    As to your original question as to "Is there a rule of thumb for duration of measurement" There is none that I have ever heard of as a regulatory period.
    There could well be a restriction on a certain design however.
    Which would be spelled out in the Manuf. literature.
    Max.
     
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